Photographic emulsion



Patented Nov. 19, 1940 UNITED STATES PATENT OFFICE rno'roormrmo EMULSION Jersey No Drawing. Application February 2, 1939, Serial No. 254,267

16 Claims.

This invention relates to photographic emulsions and more particularly to photographic emulsions, such as those of the gelatino-silverhalide type, of increased speed.

It is known that the inherent sensitivity of photographic silver halide emulsions (whether spectrally sensitized or not) can be enhanced, and there has been a constant effort to obtain higher and higher speeds in photographic emulsions. Most methods of considerably increasing sensitivity entail a sacrifice of other all important properties. Thus, to attain high speed, keeping qualities have been lowered and fog tolerances raised: For example, it has long been known that the sensitivity of photographic gelatino-silver-halide emulsions can be enhanced by bathing the emulsions (coated on plates or other suitable supports) in water or in ammonia solutions. Such a method has come to be known as hypersensitization. During the process of hypersensitization, a change in the silver ion and/or the hydrogen ion concentration in the emulsion takes place. Hypersensitized emulsions possess generally poor keeping qualities, while the method itself is highly impractical to carry out on a large scale.

It is also known that the sensitivity of gelatino-silver-halide emulsions can be enhanced by means of an extended digestion or ripening. However, such a treatment increases the fog. In an attempt to overcome this increase in fog, it has been proposed to incorporate fog retardants in the emulsion during the aforesaid extended digestion. However, such a method does of low fog content and good keeping qualities. It has also been proposed to incorporate in photographic silver halide emulsions various substances in order to enhance the sensitivity of the emulsions. For example, it has been proposed to incorporate in ordinary unsensitized silver halide emulsions or in spectrally sensitized silver halide emulsions, compounds containing a divalent atom of the sulfur gioup directly joined by, a double bond to a single metalloid atom to which at least another group of atoms is attached. Examples of such compounds are -allyl isothiocyanate and allylthiourea. However, where considerable increases of speed are attained with compounds of the aforesaid general type, the fog produced, is higher than can be tolerated in modern practice.

We have now found that the speed of photographic gelatino-silver-halide emulsions can be considerably increased by preparing the emulsions in the presence of the metal and/or ammonium thiocyanates. The keeping qualities and fog contents .of our new emulsions are within the tolerances demanded by modern practice.

We are aware that such thiocyanates have 5 been employed in photographic processes heretofore, but not in the same manner in which we employ them, nor for the same purpose. Thus, such thiocyanates have been added to finished emulsions in fairly large quantities for the purpose of toning. For example, print-out gelatinosilver-halide emulsions have been prepared by first making a solution of gelatin, water, citric acid and silver nitrate by heating the four substances together at 110 F. and then adding to such a solution another solution containing manganese chloride in quantity more than suflicient to precipitate all the silver in the first solution in the form of silver chloride. To such a finished, but unwashed emulsion, a solution of gold thiocyanate is then added. The resulting emulsion is described as a self-toning emulsion. As another example of the prior use of thiocyanates in photographic processes, we refer to the selftoning emulsions prepared by first making a solution from gelatin, water, citric acid and silver nitrate by heating the four substances together at about 110 F. and then adding to this solution another solution containing manganese chloride, shredding the. so-prepared emulsion, washing the shredded emulsion with water and then melting out at about 110 F. To such a finished, washed emulsion, a solution made from ammonium thiocyanate and sodium gold chloride is then added. As a third example of the prior use of thiocyanates in photographic processes, we refer to the self-toning emulsions prepared by adding alkali. thiocyanates, gold chlorides and dyes, such as methyl violet or fuchsin, to finished gelatino-silver-halide emulsions.

Contrasted with the foregoing processes, which always involve the addition of large quantities of thiocyanates to finished photographic emulsions, we prepare gelatino-silver-halide emul- 5 sions in the presence of the thiocyanates, i.- e. We incorporate the thiocyanates in the emulsion at some stage during the preparation of the emulsions. 7

It is an object of our invention, therefore, 59 to provide new and improved emulsions. A further object is to provide a process for preparing such emulsions. Other objects will become ap:

parent hereinafter.

halide emulsions can be prepared according to the following fundamental steps:

1. Precipitation, wherein the silver halide is precipitated by the interaction of a soluble silver salt and a soluble halide, in the presence of gelatin.

2. Digestion, wherein the above precipitate is heated for a period of time at a raised temperature, e. g. at 110 F. to 150 F., with or without the addition of further gelatin.

3. Washing, wherein the digested, solidified (e. g. by chilling) emulsion is washed with water to remove soluble salts.

4. Melting out and second digestion, wherein the washed emulsion is heated for a period of time, (with or without the addition of further gelatin) after which it is iiowable.

Of course, variations of the above four steps can be employed. An emulsion, which is used without employing a step such as (3) we call an unwashed, finished emulsion. An emulsion, which is used following completion of all four steps, we call a washed, finished emulsion. The latter is the type of emulsion generally used.

According to our invention, we incorporate metal and/or ammonium thiocyanates in the emulsion during the preparation thereof, e. g. during precipitation, during the first digestion, or during melting out and the second digestion. The thiocyanates which we have found are most advantageously employed are the alkali metal thiocyanates, such as sodium or potassium thiocyanate for example, the alkaline earth metal thiocyanates, such as calcium thiocyanate for example, and ammonium thiocyanate. Of course, thiocyanates containing cations, which cations are known to have, in themselves, a deleterious effect on silver halide emulsions should 'be avoided. Otherwise, the beneficial eflects attained by our invention would be partially nullified by the deleterious action of the cation. Thus, iron thiccyanate, which contains the iron cation, should be avoided.

The water-soluble thiocyanates are advantageously added to the emulsions in the form of their solutions in water, although they can be added in the solid form. The water insoluble thiocyanates. are advantageously added to the emulsions in the form of a colloidal dispersion in gelatin. The amount of thiocyanate which we incorporate in the emulsion can vary widely. Advantageously, however, we have employedan amount of thiocyanate equal to from about2 to about 15% by moles of the silver halide in the emulsion. Larger amounts of thiocyanate can be employed, particularly if the thiocyanate is added before washing. If the thiocyanate is not added until later in the preparation, e. g. is not added until the melting-out and second digestion step, we have found it advantageous to employ an amount of thiocyanate equal to not more than about 2% by moles of the silver halide in the emulsion. Likewise, if the emulsion is to be used unwashed, we have found it advantageous to employ not more than about 2% by moles of the" thiocyanates. Very small quantities of thiocyanate, of course, give rise to small benefits, about 0.05% being the lowest concentration at which practical benefits are obtained. If the thiocyanates are added during the first digestion and larger amounts than 2% are employed, the concentration decreases to from about 2 to 1% or less during washing. If the thiocyanates are added following the first digestion and washing, but before the second digestion of the emulsion is complete, the thiocyanates are, of course, not removed by washing, so that in either case we obtain an emulsion containing the thiocyanates in a concentration equal to not more than from about 2 to 1% or less by moles of the silver halide in the emulsion.

The emulsions which we obtain by our new process possess substantially uniformly increased sensitivity throughout the entire region of sensitivity. If we add spectrally (optically) sensitizing dyes to our new emulsions, the regions of extra sensitivity are likewise possessed of substantially uniformly increased sensitivity. It is to be noted that we cannot attain our new emulsions of substantially uniformly increased speedby merely incorporating thiocyanates in finished gelatino-silver-halide emulsions, although emulsions prepared by incorporating such thiocyanates in finished emulsions together with sensitizing dyes are useful. The sensitizing dyes, particularly those of the sensitizing cyanine dye, the sensitizing merocyanlne dye, the sensitizing hemicyanine dye and the sensitizing hemioxonol dye classes, are supersensltized in a novel and useful manner by incorporating such thiocyanates in the finished sensitized emulsions in a concentration equal to not more than about three percent by moles of the silver halide in the emulsion. See also the copending application of John A. Leermakers, Serial No. 254,252, filed of even date herewith.

While the process of our invention is subject to variation, especially as respects the particular stage of the preparation of the emulsion in which the thiocyanates are added and the nature and quantity of the thiocyanates which are employed, the following examples will serve to illustrate the manner of practicing our invention. These examples are not intended tolimit our invention.

EXAMPLE 1.Ordinary gelatino-siZver-bmmiodide emulsion Three solutions, A, B and C, were prepared as follows:

A. 60 g. of photographic gelatin were dissolved in 2000 cc. of water and 10 cc. of a 1.5 N aqueous solution of ammonium hydroxide were added.

B. 160 g. of potassium bromide and 4. g. or potassium iodide were dissolved in 900 cc. of water. a

C. 200 g. of silver nitrate were dissolved in 1200 cc. of water.

An emulsion was prepared from the above three solutions as followsi Precipitation-Solution A was heated to about 60 C. and solutions B and C, each advantageously heated to from 50 to 60 C., were simultaneously slowly added to solution A with stirring. Silver bromide precipitated.

First digestion-After addition of solutions B and C, the resulting mixture was maintained at a temperature of about 60 C. for about minutes. At the end of this time, 200 g. of gelatin were added to the mixture with stirring. The resulting-mixture was allowed to cool slowly, during a period of about 30 minutes, to about 40 C.

Washing-The mixture was then chilled for several hours until it solidified. It was then shredded and thoroughly washed with water for several minutes to remove soluble bromides.

Melting-out and second digestion-The washed emulsion was heated to from 50 to 60 C. and further gelatin was added with stirring. After addition of the gelatin, the emulsion was held at a temperature of about 60 C. for about 40 minutes and finally was allowed to cool to about 40 C. when it was readyfor coating.

sensitizing dyes of all types can be added to the emulsion prior to coating, if desired.

EXAMPLE lA.--Ordinary gelatino silver brom odiole emulsion p epared in the presence of sodium thiocyanate Three solutions, A, B and C, were prepared exactly as in Example 1, except that 4 g. of sodium thiocyanate were added to solution B. An emulsion was prepared from the three solutions precisely as in Example 1 EXAMPLE 1n.-Ordinary gelatino silver brominoliole emulsion prepared in the presence of ammonium thiocyanate Three solutions, A, B and C, were prepared exactly asin Example 1, except that 4 g. of ammonium thiocyanate were added to solution B. An emulsion was prepared from the three solutions precisely as in Example 1. v

The results obtained by preparing an ordinary gelatino-bromiodide emulsion in the presence of metal and/or ammonium thiocyanates are apparent from an inspection of the data in the following table:

EXAMPLE '2.--0Tdinary gelaiino silver chloride emulsion Two solutions, A and B, were prepared as follows:

A. 25 g. of photographic gelatin, 52 g. of sodium chloride and 5 g. of citric acid were dissolved in 2000 cc. of water.

H. 100 g. of silver nitrate was dissolved in 1000 cc. of water.

An emulsion was prepared from the above tw solutions as follows: P ecipitation-Solution A' was heated to about 60 C. and solution B, advantageously heated to from 50 to 60 C., was slowly added to solution A with stirring, maintaining the temperature of the mixture at 60 C. Silver chloride precipitated.

First digestiou.--After addition of solution B, the temperature was lowered to about 45 C. and 180 of gelatin were added .and dissolved in the mixture, with stirring at a temperature of about 45 C.

Washing.The mixture was then chilled until it solidified. It was then shredded and thoroughly washed with water to remove soluble chlorides. i

Meltino-ouL-The washed emulsion was warmed to about 40 C. when it was ready for coating' Sensitizing dyes of all types can be added to the emulsion prior to coating, if desired.

EXAMPLE 2A. Ordinary gelotino-siloer-chloride emulsion prepared in the presence of sodium thiocyanate Table II Emulsion of Example 2 Emulsion oi Example 2a H a D speed 0. 05s 27 G ii Still further examples of the preparation of our new emulsions could be given, but the foregoing are believed to demonstrate the manner of practicing our invention. Water-soluble thiocyanates are advantageously employed. When waterinsoluble thiocyanates are employed, they are advantageously incorporated in the emulsions during the melting out and second digestion step. A plurality of thiocyanates can be employed.

Photographic elements comprising our new emulsions can be made up in the usual manner by coating the flowable emulsions on to a support of a suitable material, such as glass, photographic paper, cellulose derivative or resin for example, to the desired thickness,and then drying the coated emulsion.

As stated above, sensitizing dyes of all types can be employed to spectrally sensitize our new emulsions, e. g. erythrosin, Congo red, any of the sensitizing cyanine dyes, any of the sensitizing merocyanine dyes (see United States Patent 2,078,233, dated April 27, 1937, for example), any of the sensitizing hemicyanine dyes (see the copending application of Frank L. White and Graiton H. Keyes, Serial No. 138,634, filed April 23, 1937, for example) and any of the sensitizing hemioxonol dyes (see the copending application of Grafton H. Keyes, Serial No. 146,425, filed June 4, 1937, and the copending application of Leslie G. S. Brooker, Serial No. 101,105, filed September 16, 1936, for example).

We, of course, determine the sensitivity of our new emulsions in the usual manner by first coating the prepared emulsion on a glass plate to suitable thicknessand drying the coated emulsion. The resulting photographic plate is then tested by means of a wedge spectrograph and a sensitometer, whereby spectral sensitivity and speed of the emulsion on the plate is determined.

What we claim as our invention and desire to be secured by Letters Patent of the United States is:

1. A photographic silver halide emulsion which has been subjected to a digestion step during the preparation of the emulsion, "containing a thiocyanate selected from the group consisting of metal and ammonium thiocyanates, said digestion having been conducted in the presence of said thiocyanate.

2. A photographic .gelatino-silver-halide emulsion which has been subjected to a digestion step during the preparation of the emulsion contain ing a thiocyanate selected from the group consisting of metal and ammonium thiocyanates,

said digestion having been conducted in the presence of said thiocyanate.

3. A photographic gelatino-silver-halide emulsion which has been subjected to a digestion step 7 during the preparation of the emulsion containing, in a concentration equal to not more than about two percent by moles of the silver halide inthe emulsion, a thiocyanate selected from the group consisting of metal and ammonium thiocyanates, said digestion having been conducted in the presence of said thiocyanate.

4. A photographic gelatino-silver-halide developing-out emulsion which has been subjected to a digestion step during the preparation of the emulsion containing a thiocyanate selected from the group consisting of metal and ammonium thiocyanates, said digestion having been conducted in the presence of said thiocyanate.

5. A photographic gelatino-silver-halide developing-out emulsion which has been subjected to a digestion step during the preparation of the emulsion containing, in a concentration of from about 0.05 percent to about 2 percent by moles of the silver halide in the emulsion, a thiocyanate selected from the group consisting of metal and ammonium thiocyanates, said digestion having been conducted in the presence of said thiocyanate.

6. A photographic gelatino-silver-halide developing-out emulsion which has been subjected to a digestion step during the preparation of the emulsion containing, in a concentration of from about 0.05 to about two percent by moles of the silver halide in the emulsion, an ammonium thiocyanate, said digestion having been conducted in the presence of said thiocyanate.

7. A photographic gelatino-silver halide emulsion containing a thiocyanate in a concentration equal to not more than about two percent by moles of the silver halide in the emulsion, said thiocyanate being selected from the group consisting of metal and ammonium thiocyanates.

8. A washed, finished photographic gelatinosilver-halide developing-out emulsion containing a thiocyanate in a concentration equal to not more than about two percent by moles of the silver halide in the emulsion, said thiocyanate being selected from the group consisting of metal and ammonium thiocyanates which do not contain a cation which has a deleterious effect on the emulsion.

9. A washed, finished photographic gelatinosilver-halide developing-out emulsion which has been subjected to a digestion step during the preparation of the emulsion containing, in a concentration equal to not more than about two percent by moles of the silver halide in the emulsion, a thiocyanate selected from the group consisting of metal and ammonium thiocyanates which do not contain a cation which has a deleterious effect on the emulsion, said digestion having been conducted in the presence of said thiocyanate.

10. A photographic gelatino-silver-halide developing-out emulsion containing a thiocyanate in a concentration of from about 0.05 percent'to tain a cation which has a deleterious effect on the emulsion.

12. A washed, finished photographic gelatinosilver-halide emulsion which has been subjected to a digestion step during the preparation of the emulsion containing, in a concentration equal to from about 0.05 percent to about two percent by moles of the silver halide in the emulsion, a thiocyanate selected from the group consisting of metal and ammonium thiocyanates which do not contain a cation which has a deleterious effeet on the emulsion, said digestion having been conducted in the presence of said thiocyanate.

13. A washed, finished photographic gelatinosilver-halide developing-out emulsion which has been subjected to a digestion step during the preparation of the emulsion containing, in a concentration equal to from about 0.05 to about two percent by moles of the silver halide in the emulsion, a thiocyanate selected from the group consisting of metal and ammonium thiocyanates which do not contain a cation having a deleterious effect on the emulsion, said digestion having been conducted in the presence of said thiocyanate.

14. A photographic gelatino-silver-halide developi'ng-out emulsion which has been subjected to a digestion step during the preparation of the emulsion containing, in a concentration of from about 0.05 percent to about 2 percent by moles of the silver halide in the emulsion, an alkali metal thiocyanate, said digestion having been conducted in the presence of said thiocyanate.

15. A photographic gelatino-silver-halide developing-out emulsion which has been subjected to a digestion step during the preparation of the to a digestion step during the preparation of the emulsion containing in a concentration of from about 0.05 to about two percent by moles of the silver halide in the emulsion, a thiocyanate selected from the group consisting of metal and ammonium thiocyanates which do not contain a cation which has a deleterious effect on the emulsion, said digestion having been conducted in the presence of said thiocyanate.

ADOLPH H. NIETZ. FREDERICK J. RUSSELL. 

